This invention provides an apparatus and method to measure saturated vapor/liquid mixtures used in industrial systems having various working fluids, such as in chemical, pharmaceutical, paper/pulp, petroleum and power generation industries.
The knowledge or determination of the different parameters of a process flow comprising a saturated vapor/liquid flow is used to provide feedback of the process to improve quality control of a process or detect problems or needed maintenance of the processing system. One such parameter of the vapor/liquid flow is vapor quality (e.g., steam quality) and “wetness” of the mixture. Vapor quality of a saturated vapor/liquid mixture is defined as the ratio of the mass of the vapor phase to the total mass of the mixture. Conversely, the “wetness” of a saturated vapor/liquid mixture is defined as the ratio of the mass of the liquid phase to the total mass of the mixture.
Saturated mixtures exist at temperatures and pressures at which liquid and vapor phases coexist. The temperatures and pressures at which the liquid and vapor phases coexist lie under the “vapor bubble” (i.e., saturation lines) on a phase diagram. A representative phase diagram for water is shown in FIG. 1. The collection of points known as the saturated liquid line and the collections of points known as the saturated vapor line define the vapor bubble. These two lines connect at, what is termed, the critical point. Saturated mixtures exist only under the vapor bubble. For pressures and temperatures outside of the vapor bubble, the fluid exists as a single phase and the properties of that fluid, such as density, enthalpy, internal energy, etc., are uniquely defined by the pressure and temperature. For common fluids, such as water, these properties are tabulated as functions of pressure and temperatures and are available through a variety of references including a website hosted by NIST (ref: http://webbook.nist.gov/chemistry/fluid/).
For fluids at pressures and temperatures that lie within the vapor bubble, the fluids represent mixtures of the liquid and vapor phase. Although the properties of both the vapor and liquid phases are well defined (and tabulated for known substances), the properties of the mixture are no longer uniquely defined as functions of pressure and temperature. In order to define the averaged properties of a saturated mixture, the ratio of the vapor and liquid components of the mixture must be defined. The quality of the mixture, in addition to the pressure and temperature, are defined and used to uniquely determine the properties of the mixture.
Measuring the average properties of a mixture is important in many industrial application since it is the mass averaged properties of the working fluid that enter directly into monitoring the thermodynamic performance of many processes. For example, it is the difference in the flux of enthalpy of the steam mixture flowing into and exiting from a turbine that determines the maximum mechanical work that can be extracted from the working fluid, and thus is important to determine component efficiency. However, if the steam entering or exiting the turbine were saturated, pressure and temperature measurement would not be sufficient to determine the specific enthalpy, but rather, a measurement of the quality of the steam would be required to uniquely define the thermodynamic properties of the saturated steam mixture. Note that once the quality and pressure (or temperature) of a saturated mixture is defined, the thermodynamic properties of the mixture are defined through mixing laws provided the properties of the liquid and vapor sates are known.
The present invention provides the means for measuring the speed of sound enables one to determine quality, which in turn enables one to calculate enthalpy, density, and other properties of the mixture. In addition to measuring the specific enthalpy, a measurement of the total mass is also, in general, needed to determine the flux of enthalpy.
There are many other situations where knowing the quality of a saturated mixture is beneficial. For example, in a steam power plant, the quality of the steam within the steam turbine affects blade life. Generally it is desired to operate so the quality is as high as possible throughout the turbine to minimize liquid water drops that will erode the metal blades. Knowing the quality at the turbine inlet and exhaust (or at the exhaust only if the inlet is super-heated) provides a means to monitor the quality throughout the turbine. Also, to monitor plant performance so that it can be operated at optimum conditions and to identify degradation effects, the steam turbine thermal performance must be known. This requires the fluid enthalpy at the inlet and exhaust of each turbine to be known. If the fluid at either or both locations is saturated, pressure and temperature measurements alone will not be enough to determine the enthalpy. However if an additional measurement of quality is made the enthalpy is then defined. In addition, there may be other applications in refrigeration cycles.
The ability to measure the flow rate and composition of the saturated vapor/liquid mixtures within the conduits is an important aspect of any system or strategy design to optimize the performance of a system based on saturated vapor/liquid mixtures. The industry recognizes this, and has been developing a wide variety of technologies to perform this measurement. These include probe based devices, sampling devices, venturis and ultrasonic devices